The present invention relates to an automobile power transfer apparatus for transferring to driving wheels power which is output from an engine, by way of a transmission. Further, the present invention also relates to a device for controlling torque to be transmitted to a clutch.
A power transfer apparatus such as that shown in, e.g., FIGS. 12 and 13, has hitherto been available as a power transfer apparatus of this type. FIG. 12 is a skeleton plan view of a four-wheel drive vehicle; and FIG. 13 is a semi-cross-sectional view of a power transfer apparatus. As shown in FIG. 12, the power transfer apparatus 103 is interposed between drive shafts 105, 107 in the four-wheel drive vehicle 101.
Driving force output from an engine 109 by way of a transmission and a transfer 111 is transmitted to rear wheels 117 by way of the drive shaft 105, the power transfer apparatus 103, the drive shaft 107, a rear differential 113, and an axle shaft 115. Meanwhile, the driving force is also transmitted to front wheels 123 by way of a front differential 119 and an axle shaft 121.
The power transfer apparatus 103 has such a structure as shown in FIG. 13. Specifically, a main clutch 129 serving as clutch means, a pilot clutch 131, and a cam mechanism 123 serving as conversion means are interposed between an inner rotary member 125 and an outer rotary member 127. The power transfer apparatus 103 has an electromagnet 135. The outer rotary member 127 is joined to the drive shaft 105 (see FIG. 12), and the inner rotary member 125 is joined to the other drive shaft 107 (see FIG. 12).
When the pilot clutch 131 is engaged by means of electromagnetic force derived from energization control of the electromagnet 135, the cam mechanism 123 is actuated, thereby rendering the main clutch 129 engaged. Consequently, the driving force transmitted to the drive shaft 105 is transmitted to the outer rotary member 127, and torque is transferred to the internal rotary member 125 by way of the main clutch 129, whereupon the driving force is transferred to the other drive shaft 107.
In this way, the four-wheel drive vehicle 101 shown in FIG. 12 enables transmission, to the rear wheels 117, of the driving force output from the engine 109 by way of the transmission and the transfer 111, by means of fastening force control embodied by energization control of the power transfer apparatus 103 disposed on and between the drive shafts 105, 107 (refer to, e.g., Japanese Patent Unexamined Publication No. 2000-234633, particularly, pages 2 to 4 and FIGS. 1 and 3 thereof.)
The energization control of the electromagnet 135 of the power transfer apparatus 103 is performed in accordance with an accelerator position or a throttle position of the automobile. FIGS. 14 and 15 are timing charts showing variations in target values of torque to be transmitted by the power transfer apparatus 103 (hereinafter referred to as “transmission torque”) in terms of an accelerator position or a throttle position and an A/T (automatic transmission) range. FIGS. 14 and 15 show conditions of the vehicle when switching is made from a forward traveling state to a rearward traveling state. FIG. 15 shows a state in which the four-wheel drive vehicle remains stationary by means of so-called tight corner braking phenomenon. FIGS. 14A and 15A show variations in accelerator position or throttle position, FIGS. 14B and 15B show a change in the A/T range; and FIGS. 14C and 15C show variations in target torque (control current value).
First, the four-wheel drive vehicle shown in FIG. 12 is taking corners under the stipulation that a tight corner braking phenomenon will arise. A friction factor μ existing between tires and the road surface is high. Further, the gear position of the transmission; that is, an A/T range of an automatic transmission in this example, is, for example, a drive range D. The accelerator or throttle position is in an arbitrary open position.
In such a traveling state, when the A/T range is switched from the drive range D to a reverse range R at a point in time (3) shown in FIG. 14, the accelerator or throttle position is actuated from an arbitrary open position to an arbitrary closed position at a point in time (1) (tip-out) and actuated from an arbitrary close position to an arbitrary open position at a point in time (2) (tip-in). Switching between the tip-out and the tip-in is effected within an extremely short period of time.
After the tip-out (1) has arisen, the target torque (i.e., a target value of transmission torque) of the power transfer apparatus 103 is transmitted along a line segment F1 at, e.g., a given slope. For instance, energization of the electromagnet 135 is controlled such that the target torque decreases to, e.g., transmission torque of 0 Nm.
However, when energization control is performed along such a line segment F1, the transmission torque of the power transfer apparatus 103 fails to reach a predetermined level during a duration from the tip-out (1) to the tip-in (2), the duration being an extremely short period of time. The control current again increases from a point A so as to attain the target torque corresponding to an arbitrary accelerator or throttle position.
When the gear-shifting direction of the transmission is inverted during the period existing between the tip-out (1) and the tip-in (2); for example, when switching is made from a drive position (driving forward) to a reverse position (driving backwards), with the transmission torque still remaining in the power transfer apparatus 103 in the manner as mentioned previously, there arises a problem of occurrence of an unusual noise in the power transfer apparatus 103, the transfer 111, the rear differential 113, or the front differential 119.
Specifically, when the gearing direction of the automatic transmission is inverted as a result of the automatic transmission having been switched from the drive position to the reverse position with the transmission torque still being ensured in the power transfer apparatus 103, the direction of torsion of the drive shafts 105, 107 of the drive system existing between the front and rear wheels 123, 117 is changed by means of the tight corner braking phenomenon under the foregoing driving conditions. If a change arises in the direction—in which backlash is eliminated along the axis of the power transfer apparatus 103—or the torque (force)—which would arise at the time of elimination of backlash—exceeds a predetermined level because of a change in the direction of torsion, there will arise a problem of occurrence of a collision sound grating on the occupants' ears or physical shock that the occupants find offensive.
Such collision sound or physical shock is induced by means of controlling the target torque of the power transfer apparatus 103 along the line segment F1 having a gentle slope. If the target torque is controlled along, e.g., a line segment F2 having a steep slope, the power transfer apparatus 103 can achieve a predetermined value of transmission torque before the tip-out (2). Therefore, in this case, even if a change has arisen in the direction of torsion in the drive shafts 105, 107, unusual noise which would grate on the occupants' ears will not arise as compared with the case where the transmission torque still remains in the power transfer apparatus 103.
In contrast, if the target torque of the power transfer apparatus 103 is controlled along the line segment F2 having a steep slope, vibration will arise in the vicinity of a range in which the transmission torque of the power transfer apparatus 103 assumes a value of 0 Nm. Hence, there arises a problem of the vibration being transmitted to occupants as physical shock, such as swaying motion.
According to the traveling condition illustrated in FIG. 15, the vehicle is taking corners under the stipulation that a tight corner braking phenomenon will arise, with the accelerator or throttle position being set in an arbitrary open position. The friction factor μ existing between tires and the road surface is high. Further, there will arise a situation in which the vehicle comes to a halt while taking corners by means of the tight corner braking phenomenon. In such a situation where the vehicle remains stationary, the A/T range of the automatic transmission can be considered to be switched from the drive range D to the reverse range R; i.e., inversion of the gear-shifting direction can arise. At this time, the target torque of the power transfer apparatus 103 is presumed to be set to an accelerator or throttle position required to make such a setting that the target torque exceeds an allowable level at which no unusual noise arises.
Accordingly, inversion of the gear-shifting direction arises in the same situation in which the transmission torque exists in the power transfer apparatus 103, in the same manner as described in connection with FIG. 14. The direction of torsion of the drive shafts 105, 107 is changed by the tight corner braking phenomenon in the same manner as mentioned above. Similarly, the torque developing at the time of inversion of the direction in which backlash is eliminated exceeds the allowable level at which no unusual noise arises, and hence there will arise a problem of occurrence of unusual noise (a collision sound) grating on the occupants' ears that the occupants find offensive.
The descriptions have described inversion of the gear-shifting direction by reference to the operation for switching the drive range D to the reverse range R. The inversion of gear-shifting direction arises in the same manner even by means of an operation for switching the reverse range R to the drive range D, thereby involving occurrence of unusual noise in the same manner.
In relation to the traveling state of the vehicle (i.e., a transmission state of driving force to the vehicle) and the gear-shifting state of the transmission, such as a request for high torque at a low speed of the vehicle or a request for low torque at a high speed of the vehicle, driving stability must be improved.